The sun's energy which strikes the surface of the earth is generally distributed in a wide spectral range from 350 nm to 2,100 nm and the maximum intensity of energy is found at 500 nm. When the sun's energy is divided into such spectral regions as the ultraviolet region of 449 nm or less, visible light region of 450 nm to 749 nm, and near infrared region of 750 nm to 2,100 nm, the intensity distribution of solar energy is as follows:
______________________________________ Ultraviolet region (.about.449 nm): 4.44% Visible light region (450 nm.about.749 nm): 46.3% Near infrared region (750 nm.about.2,100 nm): 49.22% Total: 100% ______________________________________
According, in controlling the incidence of solar energy, the most effective result can be obtained by preventing the incidence of solar energy in both the visible light region and near infrared region. However, a shut-out of the incidence of solar energy in the visible light region means the decrease of the intensity of light sensed by the human eye, making it dark as an inevitable consequence.
In case where a selective light transmission sheet of this invention is used on the window which forms an opening for a building, automobile, train, etc. to take in light, if the control of solar energy incidence is exercised to much as to lower the percent transmission of visible light of the window material, the proper function of a window becomes sacrificed and the house and car interiors suffer from optical hazards resulting from the remarkable loss of natural lighting visuality. Especially, the loss of visual transparency of the windshields is undesirable in view of the safety in the case of an automobile, train, and flying machine.
On the other hand, the rays in the near infrared region are not sensed by the human eye and it is accordingly possible to shut off by a maximum of about 50% of the incident solar energy while keeping the good optical environment when the per cent transmission of the rays of near infrared region is cut down effectively without controlling the per cent transmission of the visual light region.